Expand Session.TotalDuration Description

to point to the histogram with the higher maximum

Bug: 856467
Change-Id: Ie16d5190e008c7188ef4c9e4c60ef56a1c62d21f
Reviewed-on: https://chromium-review.googlesource.com/1148885
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578330}

tools/metrics/histograms/histograms.xml

@@ -91339,7 +91339,8 @@ uploading your change for review.
   <owner>chrisha@chromium.org</owner>
   <summary>
     The length of a session (launch/foregrounding to backgrounding) in
-    milliseconds.
+    milliseconds.  See Session.TotalDurationMax1Day for the same histogram with
+    a higher maximum value.  (The one's maximum is 1 hour.)
 
     Desktop: This also takes into account user interaction and audio events.
     Starting from M55 the session length is recorded by discounting for default